1. Field of the Invention
The present invention relates generally to a frequency conversion apparatus and method in a wireless communication system. In particular, the present invention relates to an apparatus and method for direct-converting a frequency in a wireless communication system.
2. Description of the Related Art
Generally, in a wireless communication system, for example, in a Code Division Multiple Access (CDMA) mobile communication system, frequency conversion is performed in order to perform wireless communication. Here, the “frequency conversion” refers to a process of converting either an externally-received high-frequency analog signal into a low-frequency signal, such as a baseband signal, such that it can be processed by the internal integrated circuit chips of a communication device, such as a portable phone, or vice versa. The conversion of a high-frequency analog signal into a baseband signal is called “frequency down-conversion”, while converting a baseband signal into a high-frequency analog signal is called “frequency up-conversion.” It will be assumed herein that the term “frequency conversion” refers to frequency down-conversion.
The frequency conversion process can be classified into a double conversion scheme and a direct conversion scheme.
Double Conversion
In a double conversion scheme, an externally-received high-frequency signal is processed twice in a radio frequency (RF) module and an intermediate frequency (IF) module. Such a scheme is called a “super-heterodyne scheme,” which is the most popular scheme. The double conversion scheme shows stable performance, and has also been popularly used for several decades.
However, a receiver using the double conversion scheme (hereafter referred to as “double-conversion receiver”) is difficult to implement. For the implementation of a double-conversion receiver, an RF chip forming an RF module, an IF chip forming an IF module, and several sub-chips included in the IF chip are required. Therefore, the use of the double conversion scheme undesirably increases the cost of a product and the required area of a board. For these reasons, it is unreasonable to use the double conversion scheme for a wireless mobile communication terminal in which its portability is emphasized. In addition, the double conversion scheme is hard to realize in a small-sized portable Multi-Band Multi-Mode (MBMM) terminal that supports multiple bands of, for example, 800 MHz and 2 GHz, and multiple modes of, for example, Personal Communications System (PCS) and Global Positioning System (GPS).
Frequency Direct Conversion
A frequency direct conversion scheme, which does not require an IF module, can remarkably reduce the number of circuit components, compared with the double conversion scheme. In addition, the frequency direct conversion scheme can solve the problems of image rejection and generation of spurious signals, possibly caused by the presence of the IF module. Despite such advantages, the frequency direct conversion scheme is difficult to use because it has it's own problems caused by the frequency direct conversion. The problems include DC offset, I/Q mismatch, inter-modulation distortion (IMD), flicker noise, and the like. Among them, the biggest problem that makes it difficult to realize a frequency direct-conversion receiver is an inflow of a DC offset, which is at a much higher level than the signal.
Therefore, much research has been conducted on methods for removing a DC offset, and a typical method for removing a DC offset is an AC coupling method. The AC coupling method, which uses a characteristic of the DC signal, uses a filter for passing only an AC signal. That is, the AC coupling method high-pass-filters a down-converted signal. Another typical method is a Time Division Multiple Access (TDMA)-based method. The TDMA-based method will now be described below in more detail with reference to FIG. 1.
FIG. 1 is a diagram illustrating a structure of a receiver used for frequency direct conversion in a TDMA system. Referring to FIG. 1, a mixer 110 mixes a high-frequency input signal with a predetermined signal to convert the high-frequency input signal into a baseband signal. The baseband signal output from the mixer 110 is input to a low-pass filter (LPF) 112, and the low-pass filter 112 low-pass-filters the baseband signal. The low-pass-filtered signal output from the low-pass filter 112 is input to an amplifier 116 via a capacitor 114. A switch 118 is connected at a contact point between the capacitor 114 and the amplifier 116. The switch 118 connects the contact point to the ground. The output signal of the capacitor 114 is input to the amplifier 116 only for a predetermined time as determined by a switching operation of the switch 118.
In TDMA, a signal is input to the receiver on a burst basis only for a specific time allocated to a user, in other words, a signal 120 is intermittently input to the receiver as illustrated in the bottom of FIG. 1, and the receiver enters an idle mode for a time when no input signal. In the idle mode, a DC offset occurring in a reception path is stored in the capacitor 114, and a value stored in the capacitor 114 is discharged during reception of data, thereby removing the DC offset.
The AC coupling method is disadvantageous in that it removes even a pure reception signal if a corner frequency is high. According to simulation results, a corner frequency of a high-pass filter (HPF) should be less than 0.1% of a data rate in order that a reduction in reception performance can be ignored in a state where there is no noise signal and frequency offset. For example, for a data rate of 48.6 bps in IS-54, a corner frequency should be 50 Hz or lower. However, in order to show a slow response to the small change in DC offset, a capacitor with a high capacitance and a resistor with a high resistance are required. Therefore, this method can be used only for a system that uses a DC-free modulation scheme in which a desired reception signal has no DC value, for example, a pager application that uses Binary Frequency Shift Keying (BFSK).
A signal with the format used in the TDMA receiver of FIG. 1 is not used in a CDMA system for the following reasons. If there is a high-level interference signal in an idle mode, a DC offset cannot be appropriately removed. In addition, because a passive element such as the capacitor 114 should be used, it is not easy to implement the receiver with highly-integrated Metal Oxide Semiconductor (MOS) transistors. Moreover, in a frequency direct-conversion receiver, when a large amount of DC offset is flowed into its lower circuit, the circuit elements are saturated or damaged undesirably.